Where to refer – orthopaedics for onward referral to physiotherapy or surgery.

Anatomy

The posterolateral corner consists of 28 individual static and dynamic structures that provide stability to the back (posterior), outer (lateral) aspect of the knee (figure 1). The lateral (fibular) collateral ligament (LCL), popliteus tendon and popliteofibular ligament are considered the most important stabilisers due to the significant support they provide to this relatively unstable part of the knee (figure 2). The common peroneal nerve is intimately related the posterolateral structures and fibula as it courses down the leg.

Causes

The LCL is the main restraint to external rotation and adduction (varus) of the tibia between 0-30° of knee flexion. The popliteus tendon provides significant resistance to external tibial rotation, while the popliteofibular ligament provides both varus and external rotation stability. Together these tissues also resist hyperextension (over-straightening) of the knee; therefore, any forceful movements into external tibial rotation, varus, hyperextension or combinations of these positions can injure the PLC structures.

Presentation

In the early stages following injury, the individual may complain of pain at the posterolateral aspect of the knee and the relevant soft tissues, or their points of insertion to bone, are usually tender on palpation (poking). Numerous PLC structures, including the LCL and popliteofibular ligament, attach to the head of the fibula and may avulse (pull away) bone during injury (figure 3). Tenderness at the fibular head may indicate a knee fracture and X-ray is therefore required to confirm or exclude this diagnosis. Pain may also be present in the medial (inside) compartment due to impaction of the bones during the injury.

Figure 3: avulsion fracture of the head of the fibula.

The LCL is an extra-articular structure (located outside the joint); therefore, injury to this ligament can result in localised lateral knee swelling. However, LCL injury rarely occurs without injury to an intra-articular structure (located inside the knee joint), therefore PLC injury usually presents with swelling within the knee joint (effusion).

Injury to the PLC can result in lateral and/or posterolateral rotatory instability (PLRI), which is often described by the individual as a ‘giving way’ sensation in the knee. Patients may demonstrate a varus thrust or posterolateral hyperextension thrust when walking, where the knee ‘thrusts’ outwards or backwards and outwards respectively during the weight bearing phase on the injured side. PLRI occurs when there is excessive posterior translation and lateral rotation of the lateral tibial plateau and the individual may walk with their lower leg and foot internally rotated to avoid placing the knee in this unstable position.

Unrecognised or untreated PLC injuries place greater strain on surgically reconstructed cruciate ligaments, which subsequently increases the risk of graft failure and further knee instability. In long-standing (chronic) cases, posterolateral knee instability can place excessive loads on the medial compartment of the knee, which in turn can lead to degenerative changes and associated medial knee symptoms.

The common peroneal nerve transmits signals from the skin to the central nervous system for sensation, and signals from the central nervous system to the muscles for muscular contraction. This nerve is affected in up to 26.2% of PLC injuries, presenting as altered sensation in the first web space (between the big toe and second toe) and/or top of the foot, or weakness into ankle dorsiflexion (upwards movements), toe extension and/or ankle eversion (outwards movement of the foot) (video 1). Individuals with weakness due to common peroneal nerve injury may walk with a foot drop gait.

Video 1: sensory and motor assessment of the common peroneal nerve.

Diagnosis

27% of PLC injuries are isolated, with most injuries occurring in combination with cruciate ligament ruptures and knee dislocations. 18% of knee dislocations involve injury to vascular structures, which can become limb or life threatening. As a priority, it is therefore important to assess for vascular injury in cases of known or suspected knee dislocations.

Clinical tests that quantify adduction (varus), posterolateral rotation of the tibia or hyperextension (recurvatum) of the knee have been recommended to assess the integrity of the PLC. For details on the diagnostic accuracy of clinical tests for PLC injury, please visit the statistics section.

Varus Stress Test

This test is performed at both 20-30° and 0° of knee flexion, assessing for lateral joint gapping/laxity and an end point (video 2).

Video 2: varus stress test at 20-30° and 0° of knee flexion.

Interpretation:

Varus laxity at 20-30°, but not at 0°, is suggestive of an LCL injury. Varus gapping at both 20-30° and 0° suggests the other PLC structures are also injured, with likely additional involvement of a cruciate ligament.

External Rotation 'Dial' Test

This test is performed with the patient supine (on their back) or prone (on their front) and knees together. The tibia is externally rotated as far as possible with the knee at 30° and 90° of knee flexion. The amount of external tibial rotation is compared between sides at both angles, using the medial borders of the feet for reference (video 3).

Video 3: prone dial test at 30° and 90°.

Interpretation:

A side-to-side difference of greater than 10° is considered a positive test. A test that is positive at 30° of knee flexion but negative at 90° is suggestive of a PLC injury, while a test that is positive at both 30° and 90° may suggest additional posterior cruciate ligament (PCL) injury.

Rationale:

The PLC and PCL work together to control external rotation of the tibia, with most resistance provided at 30° of knee flexion by the PLC; in a PLC injured knee the dial test may therefore be positive in this position. As the knee flexes (bends) further, the PCL provides more resistance to external tibial rotation. In a PLC injured knee, the amount of external tibial rotation may therefore be less at 90° when the PCL is intact, but if there is a combined PLC-PCL injury, this side-to-side difference in external rotation may remain or increase at 90°. However, the only published study that has investigated the diagnostic accuracy of the dial test found that the test can be positive at both 30° and 90° in isolated PLC injuries. Care must also be taken when interpreting the dial test as positive tests at both 30° and 90° may indicate medial knee injury often, but not always, in association with an ACL rupture.

Reverse Pivot Shift Test

This is performed with the patient supine, knee flexed to 70-80° and foot supported on the examiner’s pelvis. The foot and leg is externally rotated, an axial load is applied through the foot and a valgus force applied to the knee via the proximal fibula. The knee is then straightened, assessing for a ‘shift’ at the knee (video 4).

Video 4: reverse pivot shift test.

Interpretation:

This test aims to sublux (partially dislocate) the lateral tibia posterolaterally, which then relocates at approximately 40° of knee flexion, constituting a positive test. A positive test indicates a PLC injury with a markedly positive test suggestive of a combined PLC-PCL injury.

It is important to note that up to 35% of normal knees will test positive during the reverse pivot shift test.

External Rotation Recurvatum Test

This test is performed with the patient lying supine. The thigh is stabilised with one hand and the heel is lifted off the bed with the other hand by pulling upwards on the big toe (video 5).

Video 5: external rotation recurvatum test.

Interpretation:

The test is deemed positive if the heel lifts ≥3cm further off the bed when compared with the unaffected side and is suggestive of a combined PLC and anterior cruciate ligament injury.

Rationale:

The extra movement at the knee is caused by a combination of anterior translation, varus angulation and external rotation of the tibia.

Posterolateral Drawer Test

This test is performed with the patient supine, knee flexed to 80° and tibia externally rotated 15°. A posterolateral force is then applied to the tibia, with a finger on the posterolateral aspect of the knee assessing for laxity (video 6).

Video 6: posterolateral drawer test

Interpretation:

Increased posterolateral laxity is suggestive of a combined PLC and PCL injury.

Figure 4 and frog-leg test manoeuvre

The figure 4 test is performed with the patient supine or sitting, with the heel on the opposite leg, hip flexed, abducted and externally rotated; the leg will look like the number 4. The LCL is palpated for side to side difference.

The frog leg test is performed in supine, knees flexed to 90° and the soles of the feet together. The examiner applies a varus stress to both knees while simultaneously palpating the posterolateral structures for side-to-side difference in joint gapping and tissue integrity. This manoeuvre can be repeated at various knee angles as required (video 7).

Video 7: figure 4 and frog leg test.

Interpretation:

A side-to-side difference in joint gapping or tissue integrity constitutes a positive test, which is considered diagnostic of posterolateral instability.

Classification of PLC injuries

Hughston grading scale: the amount of lateral joint gapping is measured during a varus stress test at both 30° and 0° of knee flexion. An increase in gapping on the injured side is graded as follows (table 1):

Table 1: Hughston grading scale

Grade

I

II

III

Increased lateral knee gapping

0-5mm

5-10mm

>10mm

It is important to note that this scale is based on the perceived amount of gapping and the actual values, as measured with X-ray (varus stress radiographs), are less than proposed.

Imaging

If PLC injury is suspected, imaging can be ordered to assist diagnosis and to guide management.

The varus stress test may be performed during X-ray (varus stress radiographs), to objectively measure the difference in lateral joint gapping between sides; the amount of lateral knee gapping increases with each additional injury to a PLC structure (LCL= 2.7mm, + PLT = 3.5mm, + PFL = 4mm).

In individuals with a PCL injury, kneeling PCL stress X-rays with a side-to-side difference of more than 12mm are suggestive of a combined PCL and PLC injury.

In long-standing presentations, a standing long leg X-ray (figure 4) can identify leg malalignment that may require correction before, or during, PLC surgery. A line drawn from the head of the femur to the ankle indicates the mechanical access of the leg. If the mechanical axis falls medial to the Fujisawa point, there will more stress on the PLC, which may increase the risk of failure following PLC surgery.

MRI is useful to confirm injury to PLC structures and identify associated injuries (e.g. ACL, traumatic meniscal tears). Bone bruising may be evidence in the medial compartment of the knee, particularly the anteromedial femoral condyle, due to an impact of opposing bones.

Treatment

The optimal management of isolated PLC injuries is not conclusive as little evidence exists comparing the conservative (non-surgical) and surgical outcomes. Based on limited evidence, the outcomes of surgery are superior to non-surgical management in combined PLC and cruciate ligament injuries. Dislocated knees, which often involve the PLC, have been shown to have superior outcomes in function, knee stability, return to work and return to sport following surgical intervention.

Non-surgical/conservative management:

Early studies suggest that Grade I-II injuries may be successfully managed without surgery but grade III injuries have persistent instability and a five-fold increased chance of developing knee osteoarthritis. More recent studies have described successful management of grade III lateral collateral injuries but this evidence is limited. The PLC is initially protected with a long lever brace and protected weight bearing (figure 5) to encourage healing. For individuals with an incomplete LCL tear, a medial unloader brace (figure 6) is recommended when returning to sporting activities. Below is an example protocol, based on expert opinion and published studies.

Surgical repair/reattachment of avulsed PLC structures may only be possible within three weeks of injury as after this timeframe the injured tissue may retract or die (necrose), rendering the damaged tissues irreparable. However, in these acute presentations, the failure rate for PLC repair and staged cruciate ligament reconstruction is 38%, whereas the failure rate of PLC and cruciate ligament reconstruction is 9%. Mid-substance LCL tears, or non-acute presentations, are not considered repairable, therefore surgical reconstruction may be indicated. The failure rate for PLC reconstruction in chronic (longstanding) presentations is 10%.

Various surgical techniques have been proposed; the procedure of choice is often dependent on surgeon preference and patient presentation.

Post-operative management:

Limited evidence exists regarding rehabilitation after PLC surgery; rehabilitation is often inadequately described regarding the number of sets/repetitions of exercises and criteria for progression. Below is a recommended protocol based on expert opinion.

Foot drop:

The management of common peroneal nerve injury is dependent on the patient presentation. Most patients with an incomplete palsy (paralysis/weakness) will achieve full muscle recovery and a wait-and-see approach is therefore advocated, whereas less than 40% of patients with a complete motor palsy will regain the ability to dorsiflex at the ankle. Electromyography and nerve conduction studies may be performed to evaluate the status of the nerve and surgical intervention may be necessary if there is no evidence of recovery within 3 months of injury. During this wait-and-see period, an ankle foot orthosis (foot drop splint) and ankle range of movement exercises are required to prevent equinus deformity (stiffness resulting in an inability to dorsiflex the ankle).

Surgical intervention may involve exploration and release of the nerve from surrounding scar tissue (neurolysis) at the time of PLC repair/reconstruction, while nerve repair or nerve grafting may be considered in cases of complete nerve disruption. Based on limited evidence, the most predictable means of re-establishing antigravity dorsiflexion in persistent common peroneal nerve palsy is a posterior tibial tendon transfer.

Recovery

Recovery following PLC injury is dependent on the presence or absence of additional injury, and whether surgery has been performed or not. Return to work and sporting activity is possible in most cases after combined ACL-PLC reconstruction.

Signposting

Suspected or confirmed knee dislocations should be assessed and managed on an emergency basis (i.e. via A&E). Suspected PLC injury should be referred to orthopaedics for assessment and onward referral to physiotherapy or surgery, as appropriate.

Jakob RP, Hassler H, Staeubli HU. Observations on rotatory instability of the lateral compartment of the knee. Experimental studies on the functional anatomy and the pathomechanism of the true and the reversed pivot shift sign. Acta Orthop Scand Suppl. 1981;191:1-32.

James EW, LaPrade CM, LaPrade RF. Anatomy and biomechanics of the lateral side of the knee and surgical implications. Sports Med Arthrosc. 2015;23(1):2-9.

About the Knee Resource

The Knee Resource is founded by two clinical knee specialists Richard Norris & Daniel Massey. Together they created The Knee Resource to assist healthcare professionals to make better decisions and provide patients with information and guidance about their knee problem.